Most conventional power wheelchairs include left and right drive motors respectively connected to the left and right main wheels of the chair. A joy stick is provided as a control means, pivoting of the joy stick to the left causing the left motor to slow and the right motor to speed up and thereby turn the chair to the left. Pivoting of the joy stick to the right causes the right motor to slow and left motor to speed up to turn the chair to the right. Forward movement of the stick moves the chair forward at a speed generally proportional to the stick movement, ane rearward movement of the stick will reverse the motors to cause rearward movement of the chair.
It is desirable that the chair maintain a straight line course when the joy stick is in a centerline position; i.e., not tilted to the left or right. Normally, straight line tracking is accomplished by assuring that the left and right motors are turning at the exact same speeds. Maintaining identical speeds in turn can be accomplished by tachometer feedback circuits wherein an error signal is generated in response to any difference in the left and right motor rpm's and used to eliminate the difference.
While the foregoing controls will theoretically assure a straight line course for flat, even terrain, should there be any deviation of the terrain from a smooth flat surface, a turning of the wheelchair can result even though the main wheels are turning at the same rpm. For example, if a level surface changes to a surface sloping from the left side downwardly towards the right side, the right wheel will have to execute more rpm's than the left wheel while passing over the transition from the level surface to the sloping surface in order to keep the wheelchair on a straight course. It can be seen, accordingly, that simply maintaining identical speeds for the left and right wheels will not necessarily assure travel of the wheelchair in a straight line when the terrain is uneven.